How much more useful are ever higher pixel densities?

Definetely. Another interesting term that has been used here and there is 'true/real resolution' (google). I recently ditched a box of NEXes, the 24MP NEX-7 among them, mainly due to the lack of native lenses that are up to that sensors resolution (and extreme pickyness) - again and again: shame, shame Sony ! The linear res. of that sensor is a mere 31% over that of the NEX-5 (14MP) but that translates straight into pure unforgiveness when it comes to shooting technique in situations where you want/need all you can get from that beast.

On a side note (slightly OT): ain't that strange that a $100something 30mm Sigma outresolves each and every lens Sony has to offer for their 'flagship' NEX by some stretch, including the $1100 24mm 'Zeiss'. My hat's off to Sigma !

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'Have no fear of perfection - you'll never reach it.' Dali